In the production of glass items, particulate materials are melted in a glass furnace. The melting of the particulate material is accomplished by heating the material above its melting point by combustion of fossil fuel, by electrical heating or a combination of combustion and electrical heating. Electrical heating may be accomplished by arc melting wherein an electric arc is struck from an arc electrode to the conductive molten glass or to secondary electrodes submerged in the molten glass. Another electrical melting method is Joule heating wherein co-operating electrodes pass current between one another through the molten glass, heating the molten glass by Joule effect wherein the heating element is the resistance of the molten glass itself. In either case, electrodes protrude through the refractory of the furnace into the molten glass.
The electrodes can be constructed of any conductive material which will withstand the temperature and environment of the molten glass. Molybdenum has been found to be a good choice for electrode material. One problem with molybdenum is that it tends to oxidize at elevated temperatures. Two methods have been used to prevent oxidation. Water cooled jackets can be placed about the electrode where it enters the refractory of the furnace. This reduces the temperature of the portion of the electrode exposed to air below the point where the electrode will oxidize. The second method is to encase the hot portion of the electrode outside the furnace refractory with a purge can into which inert gas is piped. Here the electrode can be at an elevated temperature, but oxidation is prevented by exclusion of oxygen. In either case, the glass surrounding the electrode where it passes through the refractory is allowed to solidify forming a seal preventing molten glass from escaping from any clearance between the electrode and the refractory of the furnace.
This arrangement is fine for normal operation, but electrodes are consumed, wear and periodically need to be adjusted by inserting them further into the furnace. The solidified glass between the electrode and refractory of the furnace prevents the electrode from being merely pushed into the furnace. One current method of inserting electrodes into the furnace is to remove the cooling or purge can and heating the electrode with an open flame. This heat, hopefully, is sufficient to melt or at least soften the solidified glass about the electrode allowing the electrode to be pushed further into the furnace. This process is not always successful especially with higher melting point glasses, because the electrode must be kept below the oxidation temperature of the electrode.
A new and improved method and apparatus is needed to heat the electrodes to a higher temperature to facilitate adjusting the electrodes into the furnace without the risk of oxidizing the electrode.